1. Field of the Invention
This invention relates to an improvement of determining a road-wheel vibration of an automotive vehicle that is caused by a road surface condition, and especially, to an anti-skid braking system that executes a skid control procedure depending on the road-wheel vibration determined by a road-wheel vibration determining system.
2. Description of the Related Art
As is generally known, there have been proposed various anti-skid braking systems for automotive vehicles (hereinafter, referred to as xe2x80x9cABSxe2x80x9d) that prevent a skid of the road wheel during a braking in order to provide a maximum effective braking. The ABS executes a skid control by regulating a pressure of each wheel-brake cylinder mounted to each of the road wheels, depending on a pressure-reduction threshold that is calculated based on a wheel speed of each of the road wheels in a vehicle traveling condition. For instance, when the wheel speed becomes lower than a pressure-reduction threshold value, that is, the road wheel tends to be locked, the ABS executes a pressure-reduction procedure of the wheel-brake cylinder pressure. In contrast, when the wheel speed becomes higher than the pressure-reduction threshold value, the ABS executes a pressure build-up procedure so as to adjust the wheel speed toward the pressure-reduction threshold value. By repeating the pressure-reduction procedure and the pressure build-up procedure, a minimum braking distance can be provided. When skid control is executed in a bad road surface condition, the bad road surface condition causes a road-wheel vibration. Thus, if the ABS executes the skid control depending on the same pressure-reduction threshold value that is calculated in a normal road surface condition, the wheel speed fluctuates around the pressure-reduction threshold at a high-frequency that causes a frequent execution of the pressure-reduction procedure. Thereby, a braking distance tends to be long.
As a measure of the foregoing problem, there have been proposed another type of anti-skid braking system that sets the pressure-reduction threshold at a lower value as compared to a normal value set in the normal road surface condition when the bad road surface condition is determined. According to a conventional manner, the bad road surface condition is determined when the wheel speed cycle becomes lower than a predetermined value.
However, during the skid control, the road-wheel vibration is sometimes caused by a drive-train vibration. In this case, when the bad road surface condition is determined by the conventional manner, the drive-train vibration might be determined to be a road-wheel vibration that is caused by the bad road surface condition. Particularly, in a low coefficient of friction of the road surface condition (hereinafter, a coefficient of friction of the road surface is referred to as xe2x80x9cxcexcxe2x80x9d), if the pressure-reduction threshold is set at the lower value due to such determination, the pressure-reduction procedure tends to be executed late as a result of setting the pressure-reduction threshold at the lower value. Thereby, a skid of the road wheel might occur. Furthermore, since a range of a pressure-reduction volume and a pressure build-up volume tend to be large because of such late execution of the pressure-reduction procedure, a hunting of an execution of the skid control may occur.
A torque of an engine is transmitted from an engine to driving wheels via a drive shaft and a differential gear. When the ABS executes the pressure build-up procedure and the pressure-reduction procedure while the torque is accelerating the automotive vehicle, a phase shift of the torque is sometimes made due to a generation of an interference of the torque among the engine, the drive shaft, the differential gear and the driving wheel that causes a resonance of 6-7 Hz. Furthermore, in case of a four-wheel drive vehicle, which has a strong constraint force between a front axle and a rear axle, if an inverse phase of the torque is generated by an inverse timing of executing the pressure build-up procedure between the front axle and the rear axle, a torsional resonance is generated on the drive shaft.
In order to solve aforementioned problem, there has been proposed the another type of ABS having a vibration determining system that determines whether the road-wheel vibration is caused by the bad road surface condition, or the road-wheel vibration is caused by the drive-train vibration. One such vibration determining system is disclosed in a Japanese patent publication (koukai) No. 7-257347. According to the publication, the road-wheel vibration is detected based on a wheel acceleration cycle and a wheel acceleration amplitude, and then, the road-wheel vibration is determined whether the road-wheel vibration is caused by the bad road surface condition, or the road-wheel vibration is caused by the drive-train vibration based on a wheel acceleration. When the ABS determines that the road-wheel vibration is caused by the bad road surface condition, the ABS sets the pressure-reduction threshold at the lower value as compared to the normal value set in the normal road surface condition. Thereby, a tendency of the road wheel to be locked caused by the late execution of the pressure-reduction proceeding is avoided. When a drive-train vibration is identified, the pressure-reduction threshold is not changed.
However, since a change of the wheel speed varies in a bad road surface condition, the same wheel acceleration generated in a case where the drive-train vibration is causing the road-wheel vibration is sometimes made in the bad road surface condition. In this case, according to the conventional manner, since the road-wheel vibration is determined based on the wheel acceleration, the ABS determines that the road-wheel vibration is caused by the drive-train vibration, and thus, the pressure-reduction threshold is not changed. Thereby, an insufficient braking force may be made.
In case of the four-wheel drive vehicle, during a traveling in a four-wheel drive state, the ABS might determine that the road-wheel vibration is caused by the bad road surface condition, when the torsional resonance is generated on the drive shaft by a braking. As a result of such determination, the ABS sets the pressure-reduction threshold at the lower value, and the road wheel tends to be locked that causes an instability of the vehicle. Thereby, a late pressure-reduction procedure, which causes a tendency of the range of the pressure-reduction volume and the pressure build-up volume to be wide, is executed, and therefore, the hunting of the execution of the skid control that would encourage the drive-train vibration may occur. According to the related art disclosed in the publication, although the ABS sets the pressure-reduction threshold at the lower value when the bad road surface condition is determined, the minimum braking distance cannot be provided by simply setting the pressure-reduction threshold at the lower value. Moreover, although the ABS disclosed in the related art sets the pressure-reduction threshold at a higher value as compared to the normal value when the drive-train vibration is determined, the drive-train vibration cannot be inhibited due to a repeated execution of the pressure-reduction procedure and the pressure build-up procedure.
Accordingly, in view of above described problems, a principal object of the present invention is to provide an improved method of determining a road-wheel vibration and a method of executing an anti-skid control having a road-wheel vibration determining method, and an anti-skid control system using the same method, by which the road-wheel vibration is accurately determined, whether a bad road surface condition causes the road-wheel vibration or a drive-train vibration causes the road-wheel vibration.
Another object of the present invention is to provide an anti-skid braking system that provides a maximum effective braking with a minimum braking distance in a bad road surface condition.
Still another object of the present invention is to inhibit the drive-train vibration.
In order to achieve these and the other objects, a first aspect of the present invention provides a method of determining a road-wheel vibration that comprises the operations of detecting a wheel speed of each of road wheels, calculating a wheel acceleration and a wheel deceleration of each of the road wheels, calculating a wheel acceleration cycle of each of the road wheels, calculating a vehicle deceleration, detecting the road-wheel vibration based on the calculated wheel acceleration cycle, and determining whether the road-wheel vibration is caused by a bad road surface condition, or the road-wheel vibration is caused by a drive-train vibration, based on the calculated vehicle deceleration.
According to another aspect of the invention, a method of executing a skid control comprises the operations of detecting a wheel speed of each of road wheels, calculating a wheel acceleration and a wheel deceleration of each of the road wheels, calculating a wheel acceleration cycle of each of the road wheels, calculating a vehicle deceleration. Comparing the calculated wheel acceleration to a first predetermined value, and comparing the calculated wheel acceleration cycle to a second predetermined value, and in response to such comparisons, detecting a road-wheel vibration when the calculated wheel acceleration is larger than the first predetermined value, and when the calculated wheel acceleration cycle is smaller than the second predetermined value. Comparing the calculated vehicle deceleration to a third predetermined value, and in response to such comparison, determining that the road-wheel vibration is caused by a bad road surface condition when the calculated vehicle deceleration is larger than the third predetermined value, and that the road-wheel vibration is caused by a drive-train vibration when the calculated vehicle deceleration is smaller than the third predetermined value. Setting a pressure-reduction threshold depending on the operations of the detecting the road-wheel vibration and determining the road-wheel vibration. The operation of setting the pressure-reduction threshold sets the pressure-reduction threshold value at a lower value as compared to a normal road surface condition when the bad road surface condition is determined, and the operation of setting the pressure-reduction threshold sets the pressure-reduction threshold value at a higher value as compared to the normal road surface condition when the drive-train vibration is determined. And executing a skid control by regulating each wheel-brake cylinder pressure, the operation of executing the skid control executes a pressure build-up procedure, a pressure-reduction procedure, and a pressure-hold procedure depending on the pressure-reduction threshold set by said operation.
According to still another aspect of the invention, there is provided a road-wheel vibration determining system for an automotive vehicle that comprises wheel-speed sensors for detecting wheel speeds of each of road wheels, and a control unit that is configured to be electrically connected to the wheel-speed sensors for processing a wheel-speed data signal detected from each of the wheel-speed sensors. The control unit comprises a wheel acceleration calculating section for calculating a wheel acceleration and a wheel deceleration of each of the road wheels based on the wheel-speed data signal, a wheel acceleration cycle calculating section for calculating a wheel acceleration cycle of each of the road wheels, a vehicle deceleration calculating section for calculating a vehicle deceleration, a vibration detecting section for detecting a road-wheel vibration of the road wheel based on at least the wheel acceleration cycle, and a vibration determining section for determining whether the road-wheel vibration is caused by a bad road surface condition, or the road-wheel vibration is caused by a drive-train vibration of the automotive vehicle, based on the vehicle deceleration.
According to further aspect of the invention, an anti-skid braking system comprises wheel-speed sensors for detecting the wheel speed of each of road wheels, and a hydraulic brake unit for regulating wheel-brake cylinder pressure of wheel-brake cylinders mounted to each of the road wheels. The hydraulic brake unit is capable of forming a pressure build-up mode, a pressure-reduction mode, and a pressure-hold mode of each of the wheel-brake cylinders. There also is a control unit that is configured to be electrically connected to the wheel-speed sensors for processing a wheel-speed data signal detected from each of the wheel-speed sensors. The control unit comprises a wheel acceleration calculating section for calculating a wheel acceleration and a wheel deceleration of each of the road wheels based on the wheel-speed data signal, a wheel acceleration cycle calculating section for calculating a wheel acceleration cycle of each of the road wheels, a vehicle deceleration calculating section for calculating a vehicle deceleration, and a vibration detecting section for detecting a road-wheel vibration of the road wheel based on the wheel acceleration and the wheel acceleration cycle. The vibration detecting section detects the road-wheel vibration when the wheel acceleration is larger than a first predetermined value, and when the wheel acceleration cycle is smaller than a second predetermined value. There also is a vibration determining section for determining whether the road-wheel vibration is caused by a bad road surface condition, or the road-wheel vibration is caused by a drive-train vibration of the automotive vehicle. The vibration determining section determines that the road-wheel vibration is caused by the bad road surface condition when the vehicle deceleration is larger than a third predetermined value, and that the road-wheel vibration is caused by the drive-train vibration when the vehicle deceleration is smaller than the third predetermined value. There is a pressure-reduction threshold setting section for setting a pressure-reduction threshold depending on the road-wheel vibration detected and determined by the vibration detecting section and the vibration determining section. The pressure-reduction threshold setting section sets a pressure-reduction threshold value at a lower value, as compared to a normal road surface condition, when the bad road surface condition is determined by the vibration determining section, and the pressure-reduction threshold setting section sets the pressure-reduction threshold value at a higher value, as compared to the normal road surface condition, when the drive-train vibration is determined by the vibration determining section. Finally, there is a skid control section for preventing a skid of each of the road wheels by regulating each wheel-brake cylinder pressure. The skid control section executes a pressure build-up procedure, a pressure-reduction procedure, and a pressure-hold procedure depending on the pressure-reduction threshold set by the pressure-reduction threshold setting section.
According to still further aspect of the invention, an anti-skid braking system comprises wheel-speed sensors for detecting the wheel speed of each of the road wheels, and a hydraulic brake unit for regulating wheel-brake cylinder pressure of wheel-brake cylinders mounted to each of the road wheels. The hydraulic brake unit is capable of forming a pressure build-up mode, a pressure-reduction mode, and a pressure-hold mode of each of the wheel-brake cylinders. There also is a control unit that is configured to be electrically connected to the wheel-speed sensors for processing a wheel-speed data signal detected from each of the wheel-speed sensors. The control unit comprises a wheel acceleration calculating section for calculating a wheel acceleration and a wheel deceleration of each of the road wheels based on the wheel-speed data signal, a wheel acceleration cycle calculating section for calculating a wheel acceleration cycle of each of the road wheels, a vehicle deceleration calculating section for calculating a vehicle deceleration, and a vibration detecting section for detecting a road-wheel vibration of the road wheel based on the wheel acceleration and the wheel acceleration cycle. The vibration detecting section detects the road-wheel vibration when the wheel acceleration is larger than a first predetermined value, and when the wheel acceleration cycle is smaller than a second predetermined value. There is a vibration determining section for determining whether the road-wheel vibration is caused by a bad road surface condition, or the road-wheel vibration is caused by a drive-train vibration of the automotive vehicle. The vibration determining section determines that the road-wheel vibration is caused by the bad road surface condition when the vehicle deceleration is larger than a third predetermined value, and that the road-wheel vibration is caused by the drive-train vibration when the vehicle deceleration is smaller than the third predetermined value. There also is a pressure-reduction threshold setting section for setting a pressure-reduction threshold depending on the road-wheel vibration detected and determined by the vibration detecting section and the vibration determining section. The pressure-reduction threshold setting section sets a pressure-reduction threshold value at a lower value, as compared to a normal road surface condition, when the bad road surface condition is determined by the vibration determining section, and the pressure-reduction threshold setting section sets the pressure-reduction threshold value at a higher value, as compared to the normal road surface condition, when the drive-train vibration is determined by said vibration determining section. There is a skid control section for preventing a skid of each of the road wheels by regulating each wheel-brake cylinder pressure. The skid control section executes a pressure build-up procedure, a pressure-reduction procedure, and a pressure-hold procedure depending on the pressure-reduction threshold set by the pressure-reduction threshold setting section, and the skid control section prohibits the pressure build-up procedure within a predetermined period, at least right after an initial pressure-reduction procedure is executed, when the drive-train vibration is determined by the vibration determining section. The skid control section comprises a pressure build-up procedure control section for controlling the pressure build-up procedure. The pressure build-up procedure control section executes a gradual pressure build-up procedure after an initial pressure build-up procedure is executed, when said skid control section determines a further pressure build-up procedure is needed. There is a pressure-rising volume changing section for changing a pressure-rising volume depending on the road-wheel vibration detected and determined by the vibration detecting section and the vibration determining section. The pressure-rising volume changing section changes the pressure-rising volume at a higher value as compared to the normal road surface condition when the bad road surface condition is determined by the vibration determining section, and changes the pressure-rising volume at a lower value as compared to the normal road surface condition when the drive-train vibration is determined by said vibration determining section. The pressure-rising volume changing section changes the pressure-rising volume at least when the pressure build-up procedure control section executes the initial pressure build-up procedure. The pressure-rising volume changing section changes a pressure-rising period at a shorter value as compared to a normal pressure-rising period executed in the normal road surface condition when the pressure-rising volume changing section changes the pressure-rising volume at the higher value, and changes the pressure-rising period at a longer value as compared to the normal pressure-rising period when the pressure-rising volume changing section changes the pressure-rising volume at the lower value. Finally, there is a pressure-reduction volume changing section for changing a pressure-reduction volume depending on the road-wheel vibration detected and determined by the vibration detecting section and the vibration determining section. The pressure-reduction volume changing section changes the pressure-reduction volume at a lower value as compared to the normal road surface condition when the bad road surface condition is determined by the vibration determining section.